I ran my hex battery down too far last week and the battery failsafe kicked in. It is set up to autoland so down it came. The landing speed is very fast and didn't slow before the hex hit the ground. I was over deep heather so there was no damage but if the landing was on harder ground the damage would have been much worse.
I have my land speed set to a nice decent but this didn't seem to be used and the landing spot was at the same height as home so I'm a bit lost as to what is going on. Can anyone offer any pointers?
How flat was the battery? Maybe it didnt have enough power for a slow descent
Maybe, it read 10.5 volts with a battery meter but that wasn't under load.
OK, that is quite low, borderline. Rule of thumb is to ensure that battery recovers to more than 3.7 per cell off load.
If you have a log it might confirm if it was struggling if you have RCOut enabled in the logs.
You should check your flight data. Check your CURR.Volt at the end of the flight and you will probably see the voltage take a nosedive. Every time you open up your throttle, the voltage across your battery falls. As you throttle back the voltage rises again to almost the previous level.
What happens when the flightcontroller goes into RTL mode is that it throttles up to rise to safe height. As it throttles up on a battery with too low voltage, the voltage drops even lower. It tries to draw even more amps (volt * amps = power) and the voltage plummets further. It’s like opening up six big taps on a water barrel that is almost empty - the pressure, or in this case voltage disappear.
You might actually see the voltage across your battery fall to ZERO. But this is voltage under load - if you measure your battery unloaded you will see 10.5 volt.
I have done the same mistake, on a big quad puling 2800 Watts from a 6S battery. My flight data show the voltage fall to ZERO as the motors struggled to climb, while it was actually falling.
I could theoretically deplete my 6S battery to 18.6 volt, but I leave a headroom of 2 volts (i.e. 20.6 volt). This headroom is important on multicopters that draw a lot of power from the battery.